1. Field of the Invention
The present invention relates to a method of and a system for thermally recording an image or the like on a thermo-sensitive recording medium using a laser beam where the thermo-sensitive recording medium has been preheated.
2. Description of the Related Art
A thermal recording system is widespread which applies heat energy to a heat- or thermo-sensitive recording medium so as to record an image or the like thereon. In particular, a thermal recording system capable of recording an image in continuous tone at a high speed by using a laser as a heat source has been proposed (see Japanese Patent Application Laid-Open Publication Nos. 50-23617, 58-94494, 62-77983 and 62-78964, as examples).
As a thermo-sensitive recording material or medium applicable to such a thermal recording system and capable of recording satisfactory images in high quality, the present applicant has developed a medium which has leuco dyes, a developer and light-absorbing dyes all provided on a support and produces or develops color at a density corresponding to the energy of applied heat (see Japanese Patent Application Nos. 3-62684 and 3-187494, for example).
This type of thermo-sensitive recording medium has a thermo-sensitive layer formed on a support by dissolving a microcapsule containing at least leuco dyes, a developer and light- absorbing dyes in an organic solvent insoluble or soluble in water and thereafter applying a coating liquid containing particles emulsified and dispersed therein on the resultant product.
The leuco dyes have the properties for donating electrons or accepting protons such as protonic acids so as to give rise to color. The leuco dyes are normally substantially colorless and have partial skeletons such as lactone, lactam, sultone, spiropyran, ester, amide, etc. Compounds held in contact with the developer so as to allow these partial skeletons to open the ring or produce cleavage are used as the leuco dyes. Specific examples of the compounds may include crystal violet lactone, benzoyl leucomethylene blue, malachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolinobenzospiropyran, etc.
The developers corresponding to these leuco dyes, include acidic substances such as phenolic compounds, organic acids or metallic salt containing the same, oxybenzoate ester, etc. The developers may preferably have a melting point range of from 50.degree. C. to 250.degree. C. Particularly preferred is phenol or an organic acid insoluble in water having a melting point range of from 60.degree. C. to 200.degree. C. Specific examples of the developers have been described in Japanese Patent Application Laid-Open Publication No. 61-291183, for example.
Preferred as the light-absorbing dyes, are those which absorb less light in the visible range and are particularly excellent in the rate of absorption of wavelengths of radiation in the infrared region. Examples of the dyes may include cyanine dyes, phthalocyanine dyes, pyrylium and thiopyrylium dyes, azulenium dyes, squarylium dyes, metal complex dyes containing Ni, Cr, etc., naphthoquinone and anthraquinone dyes, indophenol dyes, indoaniline dyes, triphenylmethane dyes, triallylmethane dyes, aminium and diimmonium dyes, nitroso compounds, etc. Of these, those in which the absorption rate of light in the near infrared region, (with a wavelength range of from 700 nm to 900 nm) is high, are particularly preferred in view of the fact that a semiconductor laser for generating near infrared light has been put to practical use.
Since, on the other hand, the thermo-sensitive recording medium referred to above is kept in a stable conserved state, it does not develop color when the heat energy is low in level. Accordingly, a substantial quantity of heat energy is required to obtain a desired color-developed state. As a result, the dynamic range becomes narrow by a threshold value corresponding to the heat energy required to produce or develop the color and hence difficulties arise in the production of a high-tonal image. Further, a load imposed on the thermal recording system for developing the color greatly increases.
The thermo-sensitive recording medium develops colors to form a visual image depending on the thermal energy applied thereto. It is known that the density of the developed image increases with time at a normal temperature after the visual image has been recorded. Therefore, there is a problem of an image density that varies from the image density at the time immediately after recording the image, depending on the time elapsed after the recording.